What is considered by some to be the first video game — Tennis For Two
— was played on an oscilloscope at Brookhaven National Laboratory in
1958. Fifty-six years later, oscilloscopes can still play video games —
in this case, Quake.
Finnish modder Pekka Väänänen got Quake, the 1996 landmark of
PC gaming, to run on a Huawei V-422 oscilloscope. You can see it in the
video above, which shows a playthrough of Quake's first level.

Oscilloscopes measure electrical signals, so the video output here is
some kind of weird cross between vector graphics and a Pink Floyd laser
show from 1983. It's not the only piece of highly specialized
scientific equipment to run video games, either.

A more detailed technical report on how this all works may be read here.

Source.